Alternan (CAS registration No.: 136510-13-9) was described as early as 1954 by Jeanes (J. Am. Soc. 76, 5041-5052) as fraction S glucan produced extracellularly from Leuconostoc mesenteroides NRRL B-1335. Alternan is a polysaccharide composed of glucose units. The glucose units are linked with one another via alpha-1,6- and alpha-1,3 glycosidic bonds, with these two types of bond occurring predominantly alternating (Miasaki et al., 1980, Carbohydr. Res. 84, 273-285). In addition, alternan may comprise up to ca. 10% branchings (Seymour et al., 1979, Carbohydrate Research 74, 41-62). Since alternan has the alpha-1,3 glycosidic bonds occurring alternately with the alpha-1,6 in the main chain of the polymer and successive alpha-1,6 glycosidic bonds, occurring very rarely, if at all, it is differentiated from dextran, a glucose polymer consisting in the main chain of predominantly alpha-1,6 glycosidic bonds (CAS registration No.: 9004-54-0). Cote and Robyt (1982, Carbohydr. Res. 101, 57-74) therefore introduced the name alternan, which is generally valid today, for the specified fraction S glucan from Leuconostoc mesenteroides NRRL B-1335.
Native alternan has a mean molecular weight (Mw) of 106-107, is relatively readily water-soluble and confers a low viscosity on aqueous solutions (WO 03 010177).
Derivatives of alternan known hitherto are limited in that the molecular weight of native alternan has been reduced by means of physical and/or biological (enzymatic) processes.
Through the degradation of native alternan with the enzyme isomaltodextranase, a so-called limit alternan (known analogously to limit dextrins, which are obtained upon the degradation of starch by means of isoamylase) has been prepared, which has a mean molecular weight of 3500. The rheological properties (viscosity in aqueous solution) of the limit alternan correspond approximately to those of maltodextrin, i.e. limit alternan confers a low viscosity and is water-soluble in high concentrations (>80% w/v) (Cote et al., 1997, Chapter 8 In: Spanier et al. (ed), “Chemistry of novel foods”, Carol Steam, Ill.: Allured Publishing Corp., 95-110, ISBN 093171057X).
Upon incubating native alternan in the presence of fungi of the genus Penicillium, the molecular weight of the native alternan is likewise reduced (WO 03 010177). Depending on the incubation time, alternan with a molecular weight of 5-10×105 (4 days incubation time) or 1-5×104 (7 days incubation time) was obtained. Alternan-degrading enzymes could not be detected in the fungi used for this purpose, meaning that the mechanism for reducing the molecular weight of native alternan through fungi of the genus Penicillium is hitherto unexplained.
Furthermore, the average molecular weight has been reduced to less than 106 through treatment with ultrasound (Cote, 1992, Carbohydr. Polymers 19, 249-252).
Alternan with a reduced molecular weight which has been obtained following incubation with fungi or which has been obtained through treatment with ultrasound has similarities with gum arabic as regards its rheological behavior. In contrast to the solubility in water of native alternan (12%-15% w/v), it is readily water-soluble (up to 50% w/v) and confers a low viscosity on the solution. With regard to shear thinning, solutions of alternan with a reduced molecular weight exhibit a low pseudoelasticity and are approximately Newtonian liquids. In contrast to gum arabic, neither native alternan, nor alternan with a reduced molecular weight has emulsifying properties. On account of the specified rheological properties, both derivatives of alternan are proposed for the use as bulking agent, in particular in carbohydrate-containing foods (WO 03 010177).
Furthermore, alternan is a polymer which is degraded only by specific glucanases, Biley et al., 1994, Eur. J. Biochem. 226, 633-639. Such alternan-degrading glucanases are known from a few microorganisms. Consequently, alternan and alternan with a reduced molecular weight have been proposed as a low-calorie bulking agent for food products (Cote et al., 1997, Chapter 8 In: Spanier et al. (ed), “Chemistry of novel foods”, Carol Steam, Ill.: Allured Publishing Corp., 95-110, ISBN 093171057X).